How many themes do you really need? I think it is OK to repeat themes. As a matter of fact, having a set theme schedule for the next year isn't a bad idea. Leave a couple of slots open "TBD" for new ideas. It's not like people don't take new pictures that can be entered in the next go around of a specific theme. Exclude previous winners or placers in the particular theme. People can plan out their submissions. They could plan whole dive trips around entering a WPOTW contest.

Creative filters.
The need for high definition images with w/a or high macro lenses is less but the need for a good strong image prior to manipulation might encourage those with lesser cameras and lower tecnique (like me) but who occasionally play with a strong image they have managed to capture. Like this from an Aldabra channel with a 3mp Cannon G1.
Tim Digger